Turbine-driven alternators, namely turboalternators, are a key piece of machinery in many different systems for generation of energy. A turbine converts stored energy in a process gas to mechanical energy. An alternator, which is typically coupled to the turbine via a rotating shaft, converts the mechanical energy into electrical energy. The electrical energy is then supplied to a load. Such a turboalternator thus provides a means for converting energy stored in a process gas into electrical energy that is readily available to the user. These devices are especially useful for self-generation of electric energy and local power, and have long been employed in circumstances where power is not readily available from traditional sources, such as in remote locations.
U.S. Pat. No. 5,045,711 describes a prior art turboexpander-generator device typical of devices used on offshore oil/gas platforms where a source of pressurized gas is available and used to generate electricity. This device utilizes a turboexpander, an electric generator, and a lubrication pump, all fixed to a common rotating shaft. The lubrication pump provides oil to bearings supporting the rotating shaft, and further controls an actuator associated with variable inlet nozzles of the turboexpander. Unfortunately, oil-lubricated bearings as used in the device of U.S. Pat. No. 5,045,711 are unreliable, especially at the high running speeds more typical of and expected from modern turbomachinery devices. Further, shaft seals tend to wear out quickly and oil contamination of the process gas becomes a significant problem.
Similar turboalternator devices have attempted to address the deficiencies of oil-lubricated bearings. For example, U.S. Pat. Nos. 4,362,020 and 4,558,228 replace oil-lubricated bearings with hydrodynamic tilting pad bearings in energy conversion turboalternator systems. Unfortunately, titling pad bearings still suffer from high power loss, mechanical complexity, pivot fretting, limited damping capacity and indirect measurement of bearing loading.
An additional common concern with turboalternator devices is the creation of a high thermal signature. Thus, an important aspect of the design of such machinery is the creation of a temperature drop across the turbine. In turn, such a temperature drop allows the device to run more efficiently. In many conventional systems, an orifice plate is used to create a temperature drop isenthalpic expansion (i.e., the Joule-Thomson effect). By replacing the orifice with a turbine, a much higher temperature drop can be achieved, and thus more efficient operation. This occurs because high-pressure gas is expanded to produce work for driving the alternator, an isentropic process where the resultant low-pressure exhaust gas can achieve desirable very low temperature levels.
To achieve high efficiency in such machinery, the turbine must run at high rotational speeds. As rotational speed increases, the overall machine size can be made smaller without compromising the alternator's output power. Heretofore, known problems with turboalternator devices arose due to the excessive size and complexity of such devices. Requirements for running at high speed include properly designed rotating and non-rotating assemblies and bearings to support a high-speed rotating shaft, which, as noted above, permits smaller devices to be used without affecting operative efficiency and power.
Relative velocities for rotating shafts in turboalternator devices are high. For example, the running speed of the rotating shaft for a two-inch diameter turbine rotor is typically 150,000 rpm. The graph in FIG. 13 illustrates optimal operating speeds for a turbine using room temperature air as the process gas. The graph shows that a turbine with 200 psia inlet and 50 lbm/min flow should be run at 80,000 rpm to achieve optimal efficiency.
High-speed rotating machines supported on foil air bearings have made significant progress during the last 35 years. Reliability of many high-speed rotating machines with foil bearings has shown a tenfold increase compared to those with rolling element bearings. Many high-speed rotating machines are Air Cycle Machines (ACM) used in Environmental Control Systems (ECS) of aircraft that manage cooling, heating and pressurization of the aircraft. Today, ACM for almost every new ECS system on military and civilian aircraft and on many ground vehicles use foil air bearings. Old ECS systems with rolling element bearings are being converted to foil air bearings. The F-16 aircraft ACM used rolling element bearings from 1974 to 1982, but all such aircraft built since 1982 use foil air bearings. The 747 aircraft ACM used rolling element bearings from 1970 to 1989. All such aircraft built since 1989 have foil air bearings. ECS on the older model 737 aircraft have rolling element bearings, whereas ECS on the new 737 aircraft use foil air bearings. An overview of foil air bearing technology is provided in an ASME paper (97-GT-347) by Giri L. Agrawal.
The use of foil air bearings in turbomachinery has several advantages:
Oil-Free Operation—There is no contamination with oil. The working fluid in the bearing is the system process gas which could be air or any other gas. For many systems such as gas liquefaction plants, oil-free operation is a necessity.
Higher Reliability—Foil bearing machines are more reliable because there are fewer parts to support the rotating assembly and there is no lubrication needed to feed the system. When the machine is in operation, the air/gas film between the bearing and the shaft protects the bearing foils from wear. The bearing surface is in contact with the shaft only when the machine starts and stops. During this time, a coating on the foils limits the wear.
No Scheduled Maintenance—Since there is no oil lubrication system in machines that use foil bearings, there is never a need to check and replace the lubricant. This results in lower operating cost.
Environmental & System Durability—Foil bearings can handle severe environmental conditions such as shock and vibration loading.
High Speed Operation—Turbine rotors have better aerodynamic efficiency at higher speeds. Foil bearings allow these machines to operate at the higher speeds without any limitation as with ball bearings. In fact, due to the hydrodynamic action, they have a higher load capacity as the speed increases.
Low & High Temperature Capabilities—Many oil lubricants cannot operate at very high temperatures without breaking down. At low temperature, oil lubricants can become too viscous to operate effectively. Foil bearings, however, operate efficiently at severely high temperatures, as well as at cryogenic temperatures.